(1) Field of the Invention
The present invention relates to a device for feeding a welding bolt wherein the welding bolt to be welded to a sheet metal member is automatically supplied to a welding section formed between electrodes of a welder.
(2) Description of the Prior Art
In the prior art, in order that a welding bolt is welded by projection welding to a sheet member formed from a steel sheet (hereinafter referred to as "work"), a resistance welder (hereinafter abbreviated as "welder") is used. This welding method is generally performed in that a work is positioned and held onto a lower electrode of a welder and a welding bolt is inserted in a welding hole of the work and then electric power is supplied thereto under pressure by an upper electrode.
In this case, a guide hole having a larger diameter than the axial diameter of the welding bolt is bored on the lower electrode, and a guide pin movable forward or rearward by means of a spring or fluid pressure is inserted in the guide hole. The work is positioned either in a method where the work is fixed to a jig or the like attached to the welder and the welding hole corresponding to this is aligned with the axial center of the lower electrode or in a method where the welding hole of the work is fitted to the guide pin of the lower electrode.
Insertion of a welding bolt is performed in that the welding bolt aligned by a parts feeder or the like is transferred onto a lower electrode by a feed device having a grasping mechanism and is inserted by its own weight (refer to Japanese patent publication No. 127975/1984) or in that lowering action for the application of pressure by an upper electrode of the welder is utilized (refer to Japanese utility model application laid-open No. 5510/1984 and Japanese patent publication No. 54832/1985) and the welding bolt is forcedly inserted. In this connection, the feed device for forcedly inserting the welding bolt by the upper electrode exists in that the feed device is constituted integral with the upper electrode of the welder as shown in the Japanese utility model application laid-open No. 5510/1984, or in that the feed device is constituted independently from the upper electrode as shown in the Japanese patent publication No. 54832/1985.
When the welding bolt is inserted by its own weight in the welding hole of the work, however, the accurate insertion cannot be effected unless respective axial centers of the welding hole, the lower electrode and the welding bolt are aligned with each other. It is true that diameter of the welding hole of the work or the guide hole of the lower electrode is bored slightly larger than the axial diameter of the welding bolt, but deviation of the axial center (positioning deviation) must be within the dimension difference. Consequently, the higher positioning accuracy of the work is required. For example, when the positioning is performed using a jig or the like, the mechanism must be highly developed. Moreover, since such a highly developed mechanism or device is made for exclusive use on one work, it cannot be widely used for various sorts of other works and the cost of the equipment increases.
On the other hand in the above-mentioned forced inserting device wherein the feed device is integral with the upper electrode, since the upper electrode penetrates the inside of the feed device, the structure is complicated, and since the impact force is strong while the upper electrode at the bolt holder is lowered, the size becomes large in order to raise the durability of the device. Further during use, since the upper electrode is abraded at the top end plane during welding or the side surface as the bolt holder penetrates, repair or exchange of the device is frequently required. However, since the upper electrode exists in the inside of the feed device, such working becomes difficult. In addition to such problems in the mechanism, the insulation processing to prevent shorting of current during the conduction welding, the means for preventing the magnetization generated in this time and the like make the structure more complicated and of larger diameter.
Also in the apparatus where the feed device is independent from the upper electrode, although the complicated structure can be avoided, the anti-impact property to the upper electrode is small and the device as a whole is subject to deflection or vibration, whereby the direction in which the welding bolts are pushed out becomes unstable and the welding bolts may be misaligned for proper insertion in the hole. In this case, if the lowering speed of the upper electrode is decreased, the impact is reduced but the productivity of the welding deteriorates and many other problems occur.